To facilitate the interconnection of a variety of computer peripheral and terminal devices, standard interface specifications have been promulgated by various standard-setting organizations. Among the most familiar is RS232C developed by the Electronics Industries Association (EIA). This interface defines connection points for digital data signals and control ("handshaking") signals. The data signals emanating from this interface are not intended to be transmitted more than 50 feet by direct wire; for greater distances, conventional modems are usually required.
In the co-pending application of D. A. Spencer entitled "Digital Switching over a PAM Bus System," filed Sept. 1, 1981, Ser. No. 298,381, an arrangement is described which overcomes one of the problems encountered when attempting to switch digital data signals through a PBX without using modems (i.e. at baseband) at distances considerably greater than 50 feet. Advantage is sought to be taken of the comparatively high voice-sampling rate in the PBX in order to convey a PBX user's digital data signals having a lower bit rate. When such a PBX employs a PAM bus system having operational amplifiers that exhibit deliberate but loosely-controlled DC offsets, there may be some difficulty in establishing a uniform reference level for "slicing", i.e., categorizing the level of a received data signal as a "1" or "0". In the foregoing patent application each time slot was divided into a preamble interval during which the actual DC offset is measured and an active interval during which the measured offset is subtracted from the received data signal.
While a PAM bus sampling rate that is higher than a user's data signaling rate makes it conceivable for a PBX to switch the user's data at baseband without the use of voice band modems, other characteristics of the user's data equipment and signaling format must also be considered. When voice band modems are used, the PBX has the comparatively simple job of establishing a transparent channel between the modems just as if a voice call were all that was going to be completed.
PBX switching equipment can normally provide only a 2-wire switched path between its calling and called ports. Four-wire paths are normally required for full duplex operation at data rates of 1200 bits per second or greater. While "line drivers" costing less than modems can be used to transport a user's digital data over dedicated metallic paths at synchronous speeds up to 19.2 kbits per second, the use of such data communications equipment at each end of the dedicated path presupposes that the path is not switched and that only data communications equipment appears at each end of the path, i.e., there is no provision made for transporting and properly interpreting the EIA control signals that might be required to interconnect different types of data terminal equipment.
If the user is to be given the freedom of having the PBX reliably interconnect and automatically maintain the interconnection of a variety of different types of data handling equipment, either the PBX or interface equipment must undertake to verify the compatibility of the user's equipment and arrange for the continuous transport of the necessary control information presented at the interface throughout the duration of the call. For example, when the user desires to set up an intra-PBX connection between a data terminal and a computer port, the PBX must appear both to the data terminal and to the computer as if it were data communications equipment (DCE). On another call, however, the user may wish to have the PBX connect the data terminal to a remotely located computer via a modem-equipped private line. In such a case the PBX must appear as if it were data communications equipment (DCE) to the data terminal but as data terminal equipment (DTE) to the modem-equipped private line.
In each of these cases the user will also want to specify whether his data equipment is to operate in the full duplex or half duplex mode, in the synchronous or asynchronous mode and whether the data terminal or data communications equipment shall control transmission timing or whether timing is to be controlled by devices for interfacing these equipments to the PBX. In the latter case, shall timing be controlled by the local interface device clock or by the clock recovered from signals received from the remote interface device?
Data terminal equipment and data communications equipment are usually built to conform to agreed standards such as those recommended by the CCITT, the Electronics Industries Association (EIA), the International Standards Organization (ISO) and the American National Standards Institute (ANSI). CCITT recommendation V.22, for example, governing synchronous transmission at 1200 bits per second, specifies a tolerance on the bit rate of .+-.0.01%. CCITT recommendation X.21, governing synchronous transmission from 2.4 kbits per second through 48 kbits per second, contains similar tolerances. The establishment and reliable maintenance of a plurality of data communications connections within the PBX requires that the myriad encountered variations from the nominally specified bit rate be accommodated by the interface devices, i.e. that the interface devices not add to the jitter exhibited by the equipments being interconnected by the PBX.
Further, when the PBX is large enough to comprise a number of independently clocked switching modules it would be advantageous to be able to overcome the additional jitter problems occasioned by the variation in PBX voice-sampling rate among the modules.
In addition, it would be desirable for a data terminal user to be able to employ the terminal keyboard to control the PBX switching equipment to establish a data communications connection without the need for a dedicated telephone set.